1. Field of the Invention
The present invention relates to a magnetic recording medium, esp., a magnetic recording medium used with VHS system VCRs of the type that baseband-records hi-fi sound carrier signals, and to a method for making it.
2. Prior Art
A video signal comprises a luminance signal part and a chrominance or color signal part. Applied to domestic VCRs of the VHS system, etc. is a direct recording system by the low-range conversion of the chrominance signal part.
Sound carrier signal recording in the VHS system is achieved either by linear sound recording in which the sound is recorded on an end of the tape with a fixed head or by hi-fi sound recording in which the sound is recorded on a track parallel to a video signal recording track with a rotary head.
Hi-fi sound carrier signal recording is of the so-called baseband recording system in which the hi-fi sound carrier signal is first recorded, and the video signal is then recorded thereon in superimposed relation. It is noted that the recording signal of hi-fi sound is an FM modulated signal that is longer in wavelength than the luminance signal part of the video signal, as can be understood from its carrier frequencies of 1.3 MHz and 1.7 MHz. Then, the effective recording depth of the hi-fi sound carrier signal is presumed to be about 1.5 to 3 times as deep as the effective recording depth of the video signal, i.e., the effective recording depth of the chrominance signal.
In such a baseband recording system, a recording of the hi-fi sound carrier signal is partly erased by a recording of the video signal.
Hence, the hi-fi sound carrier signal output is expressed in terms of a value found by the subtraction of an output erased by the recorded video signal from a carrier ouput in the case where the video signal is not recorded in superimposed relation.
As illustrated in FIG. 1, the carrier output (1.7 MHz output), measured when the video signal is not recorded in superimposed relation, decreases with increases in the coercivity of a magnetic layer. It is noted that FIG. 1 illustrates the results of measurement in an S-VHS system.
The rate of output erasion by the recorded video signal, on the other hand, decreases in proportion to the coercivity of a magnetic layer, as illustrated in FIG. 1.
With techniques for controlling the coercivity of a magnetic layer, therefore, it is impossible to increase the hi-fi sound carrier signal (hi-fi) output to a certain or more level, as illustrated in FIG. 1.
In addition, when it is intended to increase the hi-fi sound carrier signal output to a maximum by the choice of coercivity, the best results are not always obtained for the output and S/N ratio of the video signal.
Such problems are true of the VHS and S-VHS system alike. They also arise with not only the above combination of the sound carrier signal with the video signal but also recording on a baseband recording signal a signal with the effective recording depth being smaller than that of the former.
Another problem arising with the S-VHS system in particular is that the magnetic layer tends to decrease in mechanical strength because of being formed of very fine magnetic powders. This leads to a further problem that the durability, esp., still durability or life, of video tape drops.
Accomplished with such problems in mind, the present invention has for its object to provide a magnetic recording medium which is designed such that when a surface-band recording signal is recorded on a baseband recording signal in superimposed relation, successful results are obtained for the output and S/N of the surface-band recording signal and the output and C/N of the baseband recording signal are increasingly improved, and which has improved durability, and a method for making it. Another, or a particular, object of the present invention is to provide a magnetic recording medium used with the VHS system in particular, which enables the output and C/N ratio of a hi-fi sound signal to be noticeably improved, and a method for making it.
The above and other objects of the present invention are attainable by the following aspects of the invention.
According to the first aspect of the present invention, there is provided a magnetic recording medium used with a magnetic recording system in which a baseband recording signal is recorded on a magnetic recording medium and a surface-band recording signal with the effective recording depth being smaller than that of said baseband recording signal is then recorded thereon in superimposed relation, characterized in that:
a non-magnetic substrate includes thereon a first magnetic layer and a second magnetic layer on that order,
said second magnetic layer having a thickness substantially equal to the effective recording depth of said surface-band recording signal, and
said second magnetic layer having a coercivity higher than that of said first magnetic layer.
According to the second aspect of the present invention, there is provided a magnetic recording medium as set forth in the first aspect, wherein the total thickness of said first and second magnetic layers is larger than the effective recording depth of said baseband recording signal.
According to the third aspect of the present invention, there is provided a method for making a magnetic recording medium as set forth in the first or second aspect, characterized by:
coating a magnetic coating material for said first magnetic layer on said non-magnetic substrate, followed by smoothing and drying, and then,
providing thereon another coating material for said second magnetic layer by coating.
The present invention is applied to magnetic recording systems in which a baseband recording signal is first recorded on a magnetic recording medium and a surface-band recording signal with the effective recording depth being smaller than that of said baseband recording signal is then recorded thereon in superimposed relation.
In the VHS system of the hi-fi sound baseband recording format that is a typical example of such recording systems as stated just above, the hi-fi sound carrier and video signals correspond to the baseband and surface-band recording signals, respectively.
The magnetic recording medium of the present invention comprises a substrate including thereon a first magnetic layer and a second magnetic layer on that order, said second magnetic layer having a thickness substantially equal to the effective recording depth of a video signal and a coercivity higher than that of said first magnetic layer.
Hence, it is only a part of the hi-fi sound carrier signal recorded on the second magnetic layer which is erased by the video signal recorded thereon in superimposed relation. Since the resulting rate of erasion is then very low because of the second magnetic layer having higher coercivity, the rate of reduction of the hi-fi sound carrier signal output due to superimposed recording is very low correspondingly.
In addition, partly because the first magnetic layer of lower coercivity is so highly sensitive to the hi-fi sound carrier signal and partly because the thickness of the second magnetic layer is substantially equal to the effective recording depth of the video signal, the proportion of the hi-fi sound carrier signal recorded on the first magnetic layer can be increased to a maximum.
For that reason, the present invention enables the hi-fi sound carrier signal output and C/N to be improved increasingly.
Furthermore, the luminance signal output and S/N are also greatly improved because of the second magnetic layer having higher coercivity.
Moreover, the magnetic recording medium of the present invention is improved in durability, since the magnetic layer arrangement of a double-layer structure is, as a whole, more improved in mechanical strength than a single-layer magnetic layer arrangement of similar thickness. The improvement in mechanical strength is further enhanced by providing the coating material for the second magnetic layer on the coating material for the first magnetic layer which has been dried and, preferably, cured, by means of coating.
Further features of the invention and advantages offered thereby are explained in greater detail with reference to preferred embodiments illustrated in the accompanying drawings.